Central and spinal somatosensory conduction times during hypothermic cardiopulmonary bypass and some observations on the effects of fentanyl and isoflurane anesthesia

Abstract

Somatosensory evoked potentials (SEPs) following median nerve stimulation were recorded over Erb's point (N10), neck (N13) and scalp (N20) of 17 neurologically normal patients during hypothermic cardiopulmonary bypass. Anesthesia was induced with fentanyl and 100% oxyge, and supplemented with isoflurane as necessary. All 3 SEPs were recorded at esophageal temperatures (T e) of down to 19.5°C. The central conduction time (CCT, defined as N20-N13 interpeak interval) increased exponentially with decreasing temperature (CCT T e = 1.066 37−T e·CCT 37 ; r = −0.96). The spinal conduction time (SCT, defined as N13-N10 interpeak interval) also increased exponentially but less steeply than the CCT (SCT T e = 1.047 37−T e·SCT 37 ; r = −0.89), and the N10 peak latency increased exponentially and least steeply (N10 T e = 1.033 37−T e · N10 37; r = −0.87). Anesthetic doses of fentanyl (75 μg/kg) did not affect the SEPs. Isoflurane (inspired concentration, 0.25–2.0%) produced dose-dependent increases in CCT of up to 13% and decreased N20 amplitude. All patients had normal CTs after rewarming and none suffered postoperative neurological deficits. Differences in slopes of the latency-temperature functions indicate that cooling produces more conduction slowing in central than in peripheral segments of the pathway and can be accounted for by estimates of the effects of cooling on synaptic delay and axonal conduction between wrist and cortex. The consistency of SEps between patients both during stable hypothermia and when temperature was changing suggests their potential as a sensitive monitor of cerebral status during hypothermic cardiopulmonary bypass.